Single-Crystal Diffraction at the High-Pressure Indo-Italian Beamline Xpress at Elettra, Trieste

$Single-Crystal Diffraction at the High-Pressure Indo-Italian Beamline Xpress at Elettra, Trieste$ Journal of Synchrotron Radiation research papers Single-crystal diffraction at the high-pressure Indo-Italian beamline Xpress at Elettra, Trieste. Authors Paolo Lottiab, Sula Milania, Marco Merlinia*, Boby Josephcb, Frederico Alabarseb and Andrea Lausib aDipartimento di Scienze della Terra, Università degli Studi di Milano, Via Botticelli, 23, Milano, 20133, Italy b Elettra Sincrotrone Trieste S.c.P.A., Strada Statale 14, km 163.5, Basovizza, Trieste, 34149, Italy c Research Group IISc-ICTP, Strada Statale 14, km. 163.5, Basovizza, Trieste, 34149, Italy Correspondence email: [email protected] Synopsis The experimental setup of the Xpress beamline at the Elettra synchrotron (Trieste, Italy) is described, along with the procedures for single-crystal centring and XRD data collection and processing. The reported results demonstrate the possibility to perform in situ high-pressure single-crystal X-ray diffraction experiments at the Xpress beamline. Abstract In this study we report the first in situ high-pressure single-crystal X-ray diffraction experiments at Xpress, the Indo-Italian beamline of the Elettra synchrotron, Trieste (Italy). A description of the beamline experimental setup and of the procedures for single-crystal centring, data collection and processing, using diamond anvil cells, are provided. High-pressure experiments on a synthetic crystal of clinoenstatite (MgSiO3), CaCO3 polymorphs and a natural sample of leucophoenicite (Mn7Si3O12(OH)2) validated the suitability of the beamline experimental setup to: 1) locate and characterize P-induced phase transitions; 2) solve ab initio the crystal structure of high-P polymorphs; 3) perform fine structural analyses at the atomic scale as a function of pressure; 4) disclose complex symmetry and structural features undetected using conventional X-ray sources. Keywords: Elettra synchrotron; Xpress beamline; single-crystal XRD; high pressure. 1. Introduction IMPORTANT: this document contains embedded data - to preserve data integrity, please ensure where possible that the IUCr Word tools (available from http://journals.iucr.org/services/docxtemplate/) are installed when editing this document. 1 Journal of Synchrotron Radiation research papers High-pressure diffraction grew, during the last decades, from being a niche pioneering method to a widely used multidisciplinary technique. In Earth sciences, in situ high-pressure diffraction experiments are needed to determine accurate elastic constants of minerals, by fitting the experimental V-P data by equations of state (Duffy & Wang, 1998; Angel, 2000), as well as to explore phase transitions, shedding a light on the potential mineralogy of the Earth’s interior (Boffa Ballaran et al., 2013; Dubrovinsky et al., 2010; Dera, 2010). In materials science, high-pressure diffraction allows to disclose the relationships between the crystal structure and the physical-chemical properties of crystalline compounds and induce phase transitions to polymorphs of industrial or technological interest (e.g. McMillan, 2002; Boldyreva, 2007). Most of the problems addressed by high-pressure experiments require, to be properly understood, a structural knowledge at the atomic scale. Single- crystal diffraction at high pressure is one of the most relevant techniques for the understanding of the atomic-scale behavior of matter at non-ambient conditions. However, the limited size of the pressure chambers of diamond anvil cells (e.g. Miletich et al., 2000), the absorption of primary and diffracted X-ray radiation by the diamond anvils, and the shadowing of significant portions of the reciprocal lattice by the metallic components of the DAC, may pose severe limitations on the results achievable by in situ high-P single-crystal X-ray diffraction experiments. Synchrotron radiation allows single- crystal X-ray diffraction to be performed on very small samples, allowing these experiments in a wide range of pressures and temperatures (McMahon et al., 2013; Merlini & Hanfland, 2013; Dera et al., 2013, Dubrovinsky et al., 2010), possibly even using several samples loaded in a single diamond anvil cell (Merlini et al., 2015; Yuan & Zhang, 2017). This may be crucial, for example, when only natural or synthetic samples of very small size ( 5-20 m) are available or when the solution of complex crystal structures requires the simultaneous collection of diffraction data from at least two crystals with different orientation within the same DAC. The requirements for successful single-crystal diffraction experiments concern the stability of the X-ray source, low divergence of the beam, as well as the mechanical stability of the goniometer system. We here report the first single-crystal measurements at the Xpress beamline of the Elettra synchrotron, the Italian National Synchrotron Facility. Three examples will illustrate the feasibility of single-crystal diffraction at the beamline, with data of a suitable quality for performing structure refinements, structure solution and equation-of-state determination. These examples concern the structure refinement of low- and high-pressure polymorphs of clinoenstatite, the structure determination of the high-pressure polymorphs of CaCO3, and the equation of state of the mineral leucophenicite. 2. Beamline description The high-pressure Xpress beamline end-station makes use of the X-ray beam produced by a multipole superconducting wiggler, operating with a magnetic field of 3.5 T. Primary carbon filters act as high bandpass filters, cutting the energy below 4 keV. A primary beam splitter allows 0.5 mrad of radiation in the horizontal plane to intercept the monochromator, a liquid-nitrogen cooled Si(111) single crystal. 2 Journal of Synchrotron Radiation research papers The fixed operating energy for the end-station is 25 keV (λ 0.5 Å). The beam is focused on the sample by a toroidal Pt-coated mirror at 33 m from the source and 12 m from the sample. The final beam size on the sample is approximately 0.08 x 0.08 mm2. This size can be reduced by adding sets of secondary slits and pinholes. The end-station is equipped by a 3-axis motorized stage mounted on an omega rotation goniometer. An additional translational motorized stage allows to center the goniometer vertical axis on the X-ray primary beam. The detector is a fast online MAR345 image-plate scanner, which can be positioned at variable sample- to-detector distances, in the ranges 150-500 mm. The beamline is equipped with an on-line ruby fluorescence detector system for pressure measurement and an automatic pressure controller for membrane-type diamond anvil cells. Further information on the beamline experimental setup are in Alabarse et al. (2019). 3. Sample alignment, single-crystal X-ray diffraction data collection and data processing The sample is aligned by direct beam absorption. Assuming a reference system with orthogonal axes, where the x axis is parallel to the primary beam, y is in the horizontal plane and z in the vertical plane, motorized scans along the y and z directions, monitored by a photodiode, allows the centering of the gasket hole and, possibly, of the single crystal. Scans along the y axis, performed at variable ω angles (ω being the angle made by the goniometer with rotation axis parallel to z) allow to compute the displacement of the sample from the rotation axis along the x direction and to correct accordingly. Single-crystal diffraction step scans are performed by ω step rotation, using the standard protocols for single-crystal data collections. The control software computes the goniometer motor acceleration time to reach a constant speed at the selected angle, where the fast shutter opens and the MAR image plate detector accumulates the X-ray signal during the selected ω-step rotation angle. The fast shutter closes at the end of the ω-step angle and the goniometer motor decelerates to zero speed. At this point, the MAR detector reads the image plate and data are stored on a server. Scan speed for step-scan data between 0.25 and 1 degree/second assure the best conditions for shutter synchronization, motor movement and diffraction data collection. To avoid saturation, normally Al or Fe filters are inserted in the primary beam. For single crystals of 30 µm thickness a reduction of primary beam by a factor 10 to 100 is needed, depending on the actual crystal size and scattering power of the sample. Diamond diffraction saturates the detector, but this is not a major problem with a MAR imaging plate detector. Depending on the total number of frames and detector resolution used, a data collection lasts from 0.5 to 2 hours. For example, a strategy adopting a 1° step scan, 4 sec/degree accumulation time, 60° ω-rotation and mar2300 resolution (full detector area, i.e. 345 mm in diameter, and 150x150 µm2 pixel size) requires 85 minutes. Using the mar1600 mode (150x150 µm2 pixel size and 240 mm diameter active area) the data collection lasts 45 min. Single-crystal data reduction is performed with the Crysalis software (Rigaku Oxford Diffraction 2018). 3 Journal of Synchrotron Radiation research papers 4. Single-crystal structure refinement of P21/c and C2/c clinoenstatite Clinoenstatite is the monoclinic polymorph of enstatite, MgSiO3, a mineral which represents one of the major constituents of planetary materials. The ambient-pressure P21/c structure transforms into a C2/c structure above 7 GPa at ambient temperature. The HP polymorph represents the thermodynamic stable structure of enstatite at mantle conditions. Despite the relevance of these phases, only few data on the elastic and structural behavior of HP clinoenstatite above 10 GPa exist (Lazarz et al., 2019). We report here crystal structure refinements of P21/c and C2/c clinoenstatite in the pressure interval 0-20 GPa. The results are compared with those of Lazarz et al. (2019). A synthetic single crystal of clinoenstatite was synthesized using a multi-anvil apparatus at the Earth Sciences Department of the University of Milan at ca. 6 GPa and 1200°C for 410 hours. To synthesize the clinoenstatite a gel, prepared following the procedure by Hamilton & Henderson (1968), was used as a staring material.

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